Government regulators will prevail in their battle to regulate insurer MetLife like a bank, Treasury Secretary Jack Lew said Thursday in reacting to a court opinion handing a victory to the company.

Although the government has not announced plans to appeal the federal court decision rescinding the designation of MetLife as a "systemically important financial institution," Lew said members of the Financial Stability Oversight Council "intend to continue defending vigorously the process and the integrity of [the council's] work, and I am confident that we will prevail."

Lew's statement came after the U.S. District Court for the District of Columbia unsealed the opinion in the decision handed down last week by Judge Rosemary Collyer. Both parties had to agree for the document to be released.

In the opinion, Collyer criticized the council, which is made up of the heads of the financial regulatory agencies, for being "unreasonable" in declaring that MetLife is systemically important, a label that means that it will be subject to tougher regulation. She wrote that the group had reversed itself on considering whether MetLife was subject to a run and that it hadn't adequately considered the costs of the regulations.

In a lengthy response, Lew wrote that the decision "leaves one of the largest and most highly interconnected financial companies in the world subject to even less oversight than before the financial crisis." He wrote that the crisis showed that "even the strongest firms can collapse quickly and with little warning," and that the council could not wait for a future crisis to hit to see which firms would have needed stronger regulation.

The decision was taken as a significant setback for proponents of stricter financial regulation and critics of Wall Street.

It "prioritizes MetLife's interests far above the public interest and will make future financial crashes and bailouts much more likely," wrote Dennis Kelleher, head of the advocacy group Better Markets. Kelleher criticized Collyer for second-guessing the regulators and for deciding to apply cost-benefit analysis to the decision to label MetLife.

Ohio Sen. Sherrod Brown, the ranking Democrat on the Senate Banking Committee and a prominent critic of big banks, said the decision is one that could "potentially sow the seeds of another crisis that would once again expose U.S. taxpayers to risky Wall Street practices."‎